Conventionally, there is known a torque detecting device described in, for example, Patent Document 1. In the torque detecting device, an input shaft and an output shaft are coaxially coupled to each other via a torsion bar. The torque detecting device includes a first rotation angle detecting device that detects a rotation angle of the input shaft and a second rotation angle detecting device that detects a rotation angle of the output shaft. The torque detecting device detects a torque that is applied to the input shaft on the basis of a difference between the rotation angle of the input shaft, which is detected by the first rotation angle detecting device, and the rotation angle of the output shaft, which is detected by the second rotation angle detecting device, that is, on the basis of a twist angle of the torsion bar. The torque detecting device is used, for example, in an electric power steering system.